This invention relates to clutches, particularly for use in controlling the movement of a carriage, and to drive systems incorporating such clutches.
Drive systems are known in which, during operation, a driven element energizes a biasing means as it is driven in one direction from a rest position, and a clutch automatically disengages the drive after a predetermined movement of the driven element whereby the latter is returned to its rest position under the influence of the biasing means. The present invention is particularly concerned with clutches suitable for performing such automatic disengagement of the drive which are hereinafter and in the claims referred to as "recycling clutches".
One application of such a clutch is to be found in a photocopying machine such as an electrostatographic reproduction machine as described in British Patent Specification No. 995,413 which includes lamps for exposing successive portions of the document to be copied to a photosensitive surface. These lamps which are carried on a carriage are scanned slowly across the document and are then quickly returned to their initial (start-of-scan) positions, at the end of the scanning stroke. Normally the carriage is driven in one direction by means of a cable connected to the main drive of the machine through a clutch, and is returned under the influence of a spring when the clutch is disengaged.
Scanning mirror systems for use in electrostatographic copying machines have also been proposed in which a carriage carries one or more mirrors by which images of the successive portions of the document to be copied are directed onto a photosensitive surface. Normally the light is directed onto the photosensitive surface through a fixed aperture and the photosensitive surface is arranged to move past the aperture during exposure of the document. With such an arrangement it is essential that the movement of the carriage be synchronized with the movement of the photosensitive surface which is usually in the form of a drum.
Conveniently such a carriage is driven by means of a cable passing around a capstan mounted on a drive shaft and one suitable clutch comprises rotatably input and output elemtns of which the output element carries a pawl which is normally engaged by a dog on the input element. The input element is continuously driven, the pawl being released by engaging a fixed stop member once during each revolution of the clutch whereby the carriage may be returned to its start-of-scan position under the influence of a spring, the output element of the clutch rotating in the reverse direction for slightly less than one revolution when it is again picked up by the dog on the still rotating input element. With such an arrangement in order to ensure that the movement of the carriage is synchronized with that of the photoreceptor, the capstan must be of the same size as the photoreceptor drum, and rotate at the same speed as the drum. This is generally convenient where the capstan can be mounted on the axis of the photoreceptor but particularly where it is desired to arrange the capstan in a different location, space requirements may make a capstan of these proportions inconvenient.
If the capstan is to be of reduced size yet drive the carriage synchronously with the photoreceptor it must rotate faster than the photoreceptor and through more than a single revolution for each scanning movement of the carriage. One way of achieving this is an arrangement in which the drive to an optics carriage drive shaft of an electrostatographic reproduction machine is transmitted via a timing pulley rotatably mounted on a driven shaft arranged parallel to the optics drive shaft. (The capstan is fixed on the optics drive shaft.) A cable drive connects this pulley with a pulley on the optics drive shaft. Secured on the driven shaft is a pawl plate and a pawl is pivotally mounted on the timing pulley about an axis disposed radially beyond the periphery of the pawl plate. The pawl normally engages a notch in the pawl to complete the drive to the optics drive shaft. A fixed (but adjustable) pawl knock-out plate disengages the pawl plate from the notch in the pawl plate once in each driving revolution of the pulley so permitting the optics drive shaft to rotate in reverse direction under the influence of a spring connected to the capstan to return the carriage to the start-of-scan position. More than one revolution of the optics drive shaft is achieved by making the diameter of the timing pulley a whole number multiple of the diameter of the pulley on the optics drive shaft.
With the arrangement described above, the drive is transmitted through the timing element (pulley). A recycling clutch may be provided which has the above described advantages and at the same time permits the drive to be transmitted directly to the optics drive shaft without the timing element being directly loaded.
To that end, there is described and claimed in that application a recycling clutch comprising rotatable input and output elements, cooperating drive engagement means on said elements, respectively, one of which is pivotally mounted on its associated element and biased for engagement with the engagement means on the other element, a rotatable timing element adapted to be driven by said input element and arranged to come into engagement with said pivotally mounted engagement means to effect release of the clutch at intervals greater than a revolution of said input element, and means for maintaining said engagement means disengaged for a sufficient length of time to permit rotation of the output element in reverse direction under the influence of a biasing means through greater than one revolution.
In the system described above, the timing element carries the pawl and forms part of the clutch assembly. The pawl plate rotates once for each scanning movement of the carriage. This means that it is sufficient merely to knock the pawl out of the notch in the plate and allow it to ride around the periphery of the pawl plate until it drops back into the notch, after slightly less than one revolution of the timing pulley (since the pawl plate is still rotating). Stated another way, the output element of the clutch rotates (in one direction) through slightly less than one revolution during scanning and also in (reverse direction) during rescan. However, removing the timing element from the direct drive to the carriage drive shaft requires that like the capstan the output element of the clutch shall rotate through greater than a revolution during scanning and rescan. It is not sufficient in this circumstance merely to effect release of the clutch since then it would become re-engaged before rescan had been completed. This problem is solved in a clutch according to our aforesaid concurrently filed application by the provision of means for maintaining the clutch disengaged during a sufficient operational interval of time to permit the required rotation.
In a clutch as described in that application, the pivotally mounted drive engagement means suitably takes the form of a pawl mounted on the input element and biased drivingly to engage a stop or surface on the output element and it is proposed that the clutch be maintained disengaged for the required operational interval of time by the pawl engaging a ramp or cam surface.